I'm with Kitru and ZDamned on this one.

Tank DPS is very important and only good tanks seem to realize this. A tank should maximize his DPS, just like a DPS should maximize his DPS, as long as it doesn't come at a crippling compromise to his survivability or anything else.

There is no fight in the game where Paladin Stoneskin is important. You probably want to Stoneskin between phases if the boss jumped/teleported/whatever and there's nothing to hit, just because there's nothing better to do. Aside from that, the circumstances where a Paladin should cast Stoneskin is extremely few and far between. Casting Stoneskin comes at a direct compromise to tank damage. Also, a Paladin cannot Block or Parry while casting Stoneskin.

There are exceptions to every rule, and there might be obscure situations where clutch PLD Stoneskin is the best thing for the party, but for the most part it is not as valuable as the DPS he could have been doing instead.

If I saw a Paladin Stoneskinning people mid fight, I would assess the situation and ask myself "Was there a point to that at all?" and if there wasn't, I'd probably question the Paladin's decision making and his understanding of his own class.